This invention relates generally to semiconductor devices, and more particularly the invention relates to bipolar transistor devices.
The bipolar transistor is comprised of an emitter region and a collector region of the same conductivity type in a semiconductor body separated by a base region of opposite conductivity type. Conduction in the bipolar transistor is controlled by the biases across both emitter-base and the collector-base junctions.
One of the key features of the bipolar transistor is its extremely high operating speed. At present the fastest bipolar devices are fabricated in III-V semiconducting materials, (e.g., aluminum gallium arsenide/gallium arsenide) semiconductor material. However, in order to attain a high level of integration in current technology, silicon-based semiconductor material must be used. Emitter engineering has been employed in heterojunction bipolar devices in order to improve speed and current gain by using wider band-gap materials for the emitter so that higher base doping can be achieved which leads to a lower base resistance. The reduction in base resistance results in higher operating frequencies. High operating frequencies have been demonstrated in silicon/germanium heterojunction bipolar transistors due to the use of highly doped base layers with reduced base resistance. Emitter engineering with heterojunctions has improved device performance but requires more complex device growth and processing.
Delta doped layers have been used in transistor devices to improve speed. A delta doped layer is a thin (e.g., 50 .ANG.) highly doped layer with a dopant concentration of 10.sup.13 or 10.sup.14 atoms per square centimeter, for example. The delta layer may be formed using molecular beam epitaxy or other advanced epitaxial techniques. Kuo et al. "Planarized Be Delta Doped Heterostructure Bipolar Transistor Fabricated Using Doping Selective Contact and Selective Hole Epitaxy," Japanese Journal of Applied Physics, Vol. 30, No. 2B, February, 1991, pp. 262-265, discloses a bipolar transistor in which a single p+ doped delta layer in an undoped gallium arsenide layer of 100 .ANG. thickness forms the base of a heterojunction bipolar transistor. Operating speed of the device is increased due to the thinness of the delta doped based layer.